The present invention relates to a radio communication apparatus and transmit power control method.
In a CDMA (Code Division Multiple Access) mobile communication system, transmit power control is an important technique in increasing a system capacity. One of transmit power control methods is an outer loop transmit power control method. In the outer loop transmit power control method, in order to hold the received channel quality (hereinafter abbreviated as xe2x80x9cqualityxe2x80x9d) such as FER (Frame Error Rate) at a constant level, a mobile station apparatus and base station apparatus vary a reference SIR (Signal to Noise Ratio) corresponding to the quality varying with the propagation environment, and according to a compared result between the variable reference SIR and received SIR, perform the transmit power control. In addition, the reason why the mobile station apparatus and base station apparatus vary the reference SIR so as to hold the quality at a constant level is because transmission With excessive quality increases interference to other stations, and therefore it is necessary to transmit signals with minimum transmit power meeting a required quality.
A conventional radio communication apparatus that performs the outer loop transmit power control will be described below. FIG. 1 is a principal block diagram illustrating a schematic configuration of a receiving system in the conventional radio communication apparatus. In FIG. 1, demodulating section 11 performs the predetermined demodulation processing on a received signal. Decoding section 12 decodes the demodulated data. As a decoding method, soft-decision Viterbi decoding is used for speech communications, while turbo decoding is used for data communications.
CRC decision section 13 performs CRC (Cyclic Redundancy Check) on the decoded data, decides whether or not data has an error, and outputs the decided received data. When an error is detected (when CRC is not good: CRC=NG), increment/decrement value calculating section 14 generates an increment value S1 in reference SIR expressed by the following equation (1). When an error is not detected (when CRC is good: CRC=OK), increment/decrement value calculating section 14 generates a decrement value S2 in reference SIR expressed by the following equation (2).
S1=0.5[dB]xe2x80x83xe2x80x83(1)
S2=xe2x88x92S1*FER_TARGET/(1xe2x88x92FER_TARGET)[dB]xe2x80x83xe2x80x83(2)
FER_TARGET=10xe2x88x923 (Speech communications)
FER_TARGET=10xe2x88x924 (Data communications)
The following equations (1) and (2) are such equations that are generally used as equations for calculating an increment or decrement value in reference SIR in the outer loop transmit power control. In addition, FER_TARGET is indicative of a frame error rate (quality) which the radio communication apparatus holds as a constant level.
Reference SIR updating section 15 adds an increment or decrement value in reference SIR determined by the above equation (1) or (2) and a current reference SIR value to update the reference SIR value. Generally, an update interval is 10 ms. The updated reference SIR value is output to comparing section 17.
Comparing section 17 compares an SIR value of a received signal measured in SIR measuring section 16 with the updated reference SIR value, and outputs the compared result to transmit power control bit generating section 18. When the measured SIR value is more than the reference SIR value, transmit power control bit generating section 18 generates a transmit power control bit for instructing a communication partner to decrease the transmit power. Meanwhile, when the measured SIR value is equal to or less than the reference SIR value, transmit power control bit generating section 18 generates a transmit power control bit for instructing a communication partner to increase the transmit power.
Transmit power control bit generating section 18 outputs the generated transmit power control bit to a transmitting system of the radio communication apparatus. In the transmitting system, the transmit power control bit is mapped in a transmission signal. The commination partner adjusts the transmit power according to the received transmit power control bit. The conventional radio communication apparatuses thus perform the outer loop transmit power control there between.
However, there are problems with the conventional radio communication apparatus and outer loop transmit power control method, as follows.
In turbo decoding used in data communications, decoding reliability on a decoded bit is added to a data sequence decoded by Soft Output Viterbi algorithm (SOVA) or Maximum A-Posteriori Probability algorithm (MAP). The data sequence added the decoding reliability undergoes iterated decoding, thereby increasing the decoding accuracy. In addition, the number of times the iterated decoding is performed in turbo decoding is called the number of iterations. The number of iterations is generally set to xe2x80x9c8xe2x80x9d.
In the conventional radio communication apparatus, turbo decoding is repeatedly performed always until the number of iterations reaches xe2x80x9c8xe2x80x9d. Then, according to the CRC decision result on the decoded data processed for the eighth iteration, the increment or decrement value in reference SIR is calculated. Accordingly, transmit power such that an error is not detected at the eighth iteration for the first time is optimal transmit power to increase the system capacity. In other words, transmit data with an excessive quality transmitted with excessive transmit power such that an error is not detected at the seventh or less iteration for the first time increases the interference to other stations, and thereby decreases the system capacity.
Even when the conventional radio communication apparatus receives data (data with an excessive quality) such that an error is not detected, for example, at the fourth iteration, the apparatus fixedly calculates the decrement value S2 in reference SIR, in the same way as the case of receiving data (data with an optimal quality) such that an error is not detected at the eighth iteration, according to the above equation (2). Specifically, the decrement value S2 in reference SIR is as follows:
S2=xe2x88x920.5*10xe2x88x924/(1xe2x88x9210xe2x88x924)≈xe2x88x920.5/10000[dB]xe2x80x83xe2x80x83(3)
In addition, the calculation obtains FER_TARGET=10xe2x88x924 (for data communications). It is understood from the equation that the reference SIR value decreases by an extremely gentle gradient not depending on the quality of received data.
Accordingly, in the conventional radio communication apparatus, when the quality of received data is once excessive, an extremely long time is required for the reference SIR value to be set to an optimal reference SIR value. In other words, in the conventional radio communication apparatus, a period of time data is transmitted with excessive transmit power is extremely long, thereby increasing the interference to other stations and decreasing the system capacity.
It is an object of the present invention to provide a radio communication apparatus and transmit power control method capable of preventing the decrease in a system capacity caused by excessive transmit power.
The inventors of the present invention focused on the relationship between a received channel quality and the number of times turbo decoding is iterated (the number of iterations), found out that a condition of the received channel quality is determined by the number of iterations, and carried out the present invention.
In order to achieve the above object, in the present invention, an increment or decrement width in a reference SIR value is varied adaptively corresponding to the number of iterations performed until a data error is not detected, whereby a period of time the reference SIR value is set to an optimal reference SIR value is shortened.